Planar inverted-F antennas (PIFAs) are commonly used in portable electronic systems (e.g., cellular telephones) due to their relatively small size, when compared with other antenna options. For example FIG. 1 illustrates a top view of a conventional PIFA 100, which is printed on a substrate 102 (e.g., a printed circuit board or PCB). PIFA 100 is formed in the top metal layer, as illustrated, and includes a conductive radiating element (or “antenna arm”) 104, a conductive shorting arm 106, and a conductive feed arm 108. A solid, conductive ground plane 120 is formed in a lower metal layer, as indicated by the dashed border of conductive ground plane 120. One or more conductive vias or plates (not illustrated) electrically interconnect a distal end 110 of the shorting arm 106 through the substrate 102 to the ground plane 120.
In order to use PIFA 100 to radiate or receive radio frequency (RF) signals, the PIFA 100 is interconnected with a signal source and/or load (e.g., a transceiver, not illustrated). More particularly, an input (or distal) end 112 of the feed arm 108 is electrically connected with a signal input transmission line (e.g., a 50-Ohm microstrip transmission line, not illustrated), which in turn is connected with the signal source/load. Generally, the impedance of the PIFA 100 and the impedance of the signal source/load are not matched. Accordingly, the input end 112 of the feed arm 108 may be tapered to compensate for the abrupt step transition between the input transmission line and the PIFA 100.
In conventional PIFAs, a solid ground plane (or a solid ground plane with small, narrow slots) having a certain size (e.g., typically >λ/4) is required to achieve antenna performance. Because the ground plane 120 consumes a substantial portion of the area of the layer in which it is included, conductive routing (e.g., the signal input transmission line and other routing) typically is printed on a different metal layer (e.g., the top metal layer or some other layer, not illustrated). Accordingly, conventional PIFAs typically include three or more metal layers. Alternatively, in a design that includes only two metal layers, routing is restricted to the top metal layer.